The present invention relates to a scanning drawing apparatus that is provided with a monitoring optical system for monitoring scanning position of a drawing beam.
The scanning drawing apparatus such as a laser photo plotter, which is relatively large and requires high drawing accuracy, is generally provided with a monitoring optical system for detecting the scanning position of the drawing beam in real time to control modulation of the drawing beam.
FIG. 4 is a developed view of an optical system of a conventional scanning drawing apparatus developed along a main scanning direction. A laser beam emitted from a light source 10 is separated by a half mirror M.sub.1 into a drawing beam L.sub.1 and a monitor beam L.sub.2. The drawing beam L.sub.1 is reflected by the half mirror M.sub.1 and a mirror M.sub.2, passes through a first reducing optical system 1, enters an acoustooptic modulator (AOM) 2, and is incident on a second reducing optical system 3. The drawing beam L.sub.1 is then reflected by a mirror M.sub.3 and is incident on a half mirror M.sub.4. The monitor beam L.sub.2, which passes through the half mirror M.sub.1, passes through a third reducing optical system 4 and is incident on the half mirror M.sub.4.
The drawing beam L.sub.1, which is reflected by the half mirror M.sub.4, and the monitor beam L.sub.2, which passes through the half mirror M.sub.4, are combined by the half mirror M.sub.4. The combined beams are deflected by a polygon mirror 6 and converged by an f.theta. lens 7. The drawing beam L.sub.1 scans an image surface (not shown) and the monitor beam L.sub.2 scans a monitor scale (not shown) that is located at a position that is geometric-optically equivalent to but apart from the image surface.
The monitor scale is a transparent plate on which a large number of opaque lines are formed in a constant pitch. When the monitor beam L.sub.2 scans the monitor scale, a photodetector, which receives light passed through the monitor scale, outputs a pulse signal. A controller of the scanning drawing apparatus counts the number of pulses from a predetermined starting point to detect the scanning position of the drawing beam in real time. Then, the controller modulates the drawing beam in accordance with the detected scanning position.
The monitor optical path between the half mirrors M, and M.sub.4 through which only the monitor beam L.sub.2 travels is shorter than the drawing optical path between the half mirrors M.sub.1 and M.sub.4 through which only the drawing beam L.sub.1 travels. It is because the drawing optical path includes the reducing optical systems 1 and 3 at both sides of the AOM 2 and requires a certain length, while the monitor optical path only includes the reducing optical system 4.
Further, when the drawing apparatus employs a multiple drawing beam system in which a plurality of drawing beams simultaneously scan the image surface, a beam separator is arranged between the half mirror M.sub.1 and the first reducing optical system 1 for dividing the incident laser beam into a plurality of parallel drawing beams, which also requires longer optical path length and a larger effective diameter of each optical element arranged in the drawing optical path.
Since the drawing optical path is different from the monitor optical path in the optical path length and/or the effective diameter of the elements arranged therein, magnifications, the number of mirrors and the like are different between the drawing optical path and the monitor optical path. Therefore, a deviation of the beam spot of the drawing beam L.sub.1 on the image surface due to an inclination or a shift of the light source 10 is not coincident with a deviation of the beam spot of the monitor beam L.sub.2 on the monitor scale in terms of directions and amounts.
For instance, when the light source 10 is at the proper position so that the optical axis of the light source 10 is coincident with the optical axis of the optical system indicated by a single-dot chain line in FIG. 4, the drawing beam L.sub.1 and the monitor beam L.sub.2 travel along the optical axis of the optical system, which does not cause the deviation between the beams.
If the light source 10 shifts in the upward direction in FIG. 4 by .delta..sub.0, the drawing beam L.sub.1 travels along a solid line and the beam spot on the image surface shifts in the downward direction by .delta..sub.1, while the monitor beam L.sub.2 travels along a dotted line and the beam spot on the monitor scale shifts in the upward direction by .delta..sub.2.
When a gas laser is used as the light source 10, the optical 10 axis thereof may shift or incline in accordance with a lapse of time after turning ON of the light source. Further, an exchange of the light source 10 due to the termination of the useful life may cause the deviation of the optical axis thereof.
The difference of the deviations between the drawing beam L.sub.1 and the monitor beam L.sub.2 makes it difficult to maintain a correlation between the monitor signal and the scanning position of the drawing beam, which disables an accurate control of the modulation.